1. Field of the Invention
This invention relates to an enclosed electric motor wherein a rotor assembly is provided in a motor casing together with dynamic pressure air bearing means.
2. Description of the Prior Art
Enclosed electric motors are provided with dynamic pressure air bearing means in which air is used as a lubricating fluid. These enclosed motors are used as scanner motors for driving polygon mirrors which are employed for scanning laser beams in laser beam printers. FIG. 7 illustrates such a conventional enclosed electric motor used for driving the polygon mirror. A base 1 of the motor has a plurality of stepped portions on the upper face thereof and a cylindrical portion 2 formed in the central bottom thereof. A bearing cylinder 3 is inserted in the cylindrical portion 2 to be fixed in position by an adhesive agent. A bottom lid 4 is screwed to the bottom of the cylindrical portion 2. A cover 5 is screwed to the upper side of the base 1 so as to cover the bearing cylinder 3. The base 1 and the cover 5 constitute an enclosed motor casing 6. An electric circuit board 7 is screwed to an upper portion of the base 1 in the motor casing 6. A plurality of stator coils 8 are fixed to the upper face of the circuit board 7 by an adhesive agent.
A rotor assembly 10 including a rotational shaft 9 is provided in the motor casing 6. The rotational shaft 9 is rotatably inserted in the bearing cylinder 3 to be held in position. The rotational shaft 9 and the bearing cylinder 3 constitute dynamic pressure air bearing means. Two pairs of herringbone grooves 11, which compose part of dynamic pressure air bearing means, are formed in the outer circumferential face of the rotational shaft 9.
A flange 12 is fixed to the upper portion of the rotational shaft 9. A rotor yoke 13 is fixed to the flange 12 by an adhesive agent. An annular rotor magnet 14 is fixed to the underside of the rotor yoke 13 by an adhesive agent. The rotor magnet 14 is disposed to be opposite to the stator coils 8 with a predetermined axial gap therebetween. A polygon mirror 15 is mounted to the flange 12. The cover 5 has a window 5a which is disposed so as to correspond to the outer periphery of the polygon mirror 15. Laser beams are allowed to go into and out of the motor casing 6 through the window 5a.
A mounting member 16 is mounted to the back of the flange 12 so as to be rotated with the rotational shaft 9. The mounting member 16 extends through the circuit board 7 so as to cover the bearing cylinder 3 from above the same. A magnetically focusing rotating yoke 17 is fixed to the lower portion of the mounting member 16 so as to be positioned under the circuit board 7. The rotating yoke 17 is disposed to be opposite to the rotor magnet 14 with the circuit board 7 and the stator coils 8 being interposed therebetween so that magnetic fluxes generated by the rotor magnet 14 pass through the rotating yoke 17. A sliding thrust receiving member 18 comprising a sliding bearing is provided on the inside of the bottom lid 4. A thrust load of the rotor assembly 10 is received by the sliding thrust receiving member 18.
Upon rotation of the rotor assembly 10, the herringbone grooves 11 draw air into a bearing gap of several .mu.m defined between the inner circumferential face of the bearing cylinder 3 and the outer circumferential face of the rotational shaft 9, whereupon high dynamic pressure serving as dynamic pressure air bearing is produced. The rotational shaft 9 is rotated without contact with the bearing cylinder 3 by the action of the established dynamic pressure air bearing. Electric motors employing the dynamic pressure air bearing as described above are suitable for high speed rotation.
The enclosed motors with the above-described dynamic pressure air bearing have the following problem. The motors of the type as described above are used in a high speed range (30,000 rpm or above, for example) in many cases. An outer configuration of the rotor assembly 10 or an inner configuration of the cover 5 of the motor casing 6 causes differences in the pressure in a space inside the cover 5. More specifically, air is forced toward the outer periphery of the rotor assembly 10 upon rotation of the same. The air then strikes against the inner inclined face 5b of the cover 5, flowing downwardly. The rotor magnet 14 of the rotor assembly 10 is disposed in the lower space inside the cover 5 and has a large radius of rotation. Accordingly, a gap between the cover 5 and the rotor assembly 10 is relatively small in the lower space inside the cover 5, in which space the rotor magnet 14 is disposed. On the other hand, a gap between the cover 5 and the rotor assembly 10 is larger in the upper space inside the cover 5, in which space the upper end of the rotor assembly 10 is located. A pumping action with rotation of the rotor assembly 10 causes air in the upper space inside the cover 5 to flow downwardly to the lower space inside the cover 5, whereupon the pressure is increased in the lower space while the upper space approaches a vacuum state. Consequently, the rotor assembly 10 is irregularly moved, for example, it is drawn upwardly. In the motor employing the dynamic pressure air bearing, particularly, such irregular movement of the rotor assembly 10 prevents the smooth rotation of the shaft 9 by the dynamic pressure air bearing.
Furthermore, the air high-pressurized in the bearing gap between the bearing cylinder 3 and the rotational shaft 9 generates heat, which increases the temperature in the bearing section. Since the shaft 9 is made of a stainless steel while the bearing cylinder 3 is made from ceramic, for example, the difference in the material between these members results in a difference in a coefficient of thermal expansion between them. The difference in the thermal expansion coefficient between the members causes the bearing gap 19 to vary with an increase in the temperature. This also prevents the normal rotation of the shaft 9 by the dynamic pressure air bearing.
Additionally, noise is produced in the space where the rotor assembly 10 is close to the polygon mirror 15, particularly, in the vicinity of the window 5a. The noise is due to rotation of the polygon mirror through the air.